Glucagon-like peptide-1 (GLP-1) is a relatively recently discovered gastrointestinal hormone (Holst, Gastroenterology 107: 1848-1855, 1994) that has attracted considerable interest because of its potent actions on carbohydrate metabolism and its potential use as a therapeutic agent in the treatment of type-2 diabetes (Gutniak et al., N. Engl. J. Med. 326: 1316-1322, 1992; Nathan et al., Diabetes Care 15: 270-276, 1992). It arises from tissue-specific processing of the glucagon precursor, proglucagon, within the L-cell of the distal intestinal mucosa (Bell et al., Nature (London) 304: 368-371, 1983; Mojsov et al., J. Biol. Chem. 261: 11800-11889, 1986; Orskov et al., Endocrinology 119: 1467-1475, 1986), from which it is secreted in response to meal ingestion (Elliott et al., J. Endocrinol. 38: 159-166, 1993; Herrmann et al., Digestion 56: 117-126, 1995; Orskov et al., Scand. J. Gastroenterol. 31: 665-670, 1996).
The processing pattern of proglucagon leading to the formation of GLP-1 is known in detail (Bell, supra; Mojsov, supra; Ørskov et al., Endocrinology 119: 1467-1475, 1986; Ørskov et al., Diabetes. 43: 535-539, 1994; Ørskov et al., Diabetologia. 30: 874-881, 1987). Initial studies of GLP-1 biological activity in the mid 1980s utilized the full length N-terminal extended forms of GLP-1 (1-37 and 1-36amide). These larger GLP-1 molecules were generally devoid of biological activity. In 1987, 3 independent research groups demonstrated that removal of the first 6 amino acids resulted in a shorter version of the GLP-1 molecule with substantially enhanced biological activity. The majority of circulating biologically active GLP-1 is found in the GLP-1(7-36)amide form), with lesser amounts of the bioactive GLP-1(7-37) form also detectable. See Orskov et al. (Diabetes 43(4): 535-9, 1994) for the human data. Both peptides appear equipotent in all biological paradigms studied to date. GLP-1 is secreted from gut endocrine cells in response to nutrient ingestion and plays multiple roles in metabolic homeostasis following nutrient absorption.
Furthermore, the two naturally occurring truncated peptides, GLP-1(7-37)OH and GLP-1(7-36)NH2, are rapidly cleared in vivo and have extremely short in vivo half-lives. In several studies (Deacon et al., J. Clin. Endocrinol. Metab. 80: 952-957, 1995; Deacon et al., Diabetes. 44: 1126-1131, 1995; Mentlein et al., Eur. J. Biochem. 214: 829-835, 1993), it was found that GLP-1, in addition to its renal and hepatic elimination (Deacon et al., Am. J. Physiol. Endocrinol. Metab. 34: E458-E464, 1996), is degraded in plasma by the enzyme dipeptidyl peptidase IV (DPP-IV). The enzyme removes a dipeptide from the N terminus of GLP-1 (Deacon, supra; Mentlein, supra), producing the truncated fragment GLP-1 (9-36)amide. This cleavage of GLP-1 seems to be relevant for its actions on the endocrine pancreas. Thus, in a recent study of the interaction of GLP-1 (9-36) amide with the cloned GLP-1 receptor, GLP-1 (9-36) amide antagonized the action of GLP-17-36 amide (Knudsen and Pridal, Eur. J. Pharmacol. 318: 429-435, 1996). In another study, after showing previously that GLP-1 strongly inhibited cephalic-induced antral motility in pigs, Wettergren et al. (Peptides 19(5): 877-882, 1998) reported that an intact N terminus is essential for the gastrointestinal actions of GLP-1, and that its primary metabolite, GLP-1 (9-36)amide, may even act as an endogenous antagonist.
Further, GLP-1 compound formulations currently in development cannot be given orally and like insulin, must be injected. Thus, despite the clear medical advantages associated with therapy involving GLP-1, the short half-life which results in a drug that must be injected one or more times a day has impeded commercial development efforts. Generally, moving patients to an injectable therapy is quite difficult. For example, many diabetics are unwilling to undertake any type of intensive injection therapy due to the discomfort associated with the many injections required to maintain adequate glucose control. Furthermore, diabetics on insulin are generally required to monitor their blood glucose, which involves additional needle sticks. This type of therapy can be both psychologically and physically painful. This is especially true when patients have been treated solely with oral medications throughout the progression of the disease.
Therefore, there is a need in the art for more effective ways to deliver GLP-1 to patients in need thereof, preferably through less-invasive delivery means, so as to overcome or at least reduce several problems associated with GLP-1 therapy, such as the potentially inhibitory function of the natural metabolites of GLP-1, and the painful and invasive delivery means.